Involvement of ABC transporters in melanogenesis and the development of multidrug resistance of melanoma

Because melanomas are intrinsically resistant to conventional radiotherapy and chemotherapy, many alternative treatment approaches have been developed such as biochemotherapy and immunotherapy. The most common cause of multidrug resistance (MDR) in human cancers is the expression and function of one or more A TP‐b inding c assette (ABC) transporters that efflux anticancer drugs from cells. Melanoma cells express a group of ABC transporters (such as ABCA9, ABCB1, ABCB5, ABCB8, ABCC1, ABCC2, and ABCD1) that may be associated with the resistance of melanoma cells to a broad range of anticancer drugs and/or of melanocytes to toxic melanin intermediates and metabolites. In this review, we propose a model (termed the ABC‐M model) in which the intrinsic MDR of melanoma cells is at least in part because of the transporter systems that may also play a critical role in reducing the cytotoxicity of the melanogenic pathway in melanocytes. The ABC‐M model suggests molecular strategies to reverse MDR function in the context of the melanogenic pathway, which could open therapeutic avenues towards the ultimate goal of circumventing clinical MDR in patients with melanoma.     

The phosphodiesterase-5 inhibitor vardenafil is a potent inhibitor of ABCB1/P-glycoprotein transporter

One of the major causes of chemotherapy failure in cancer treatment is multidrug resistance (MDR) which is mediated by the ABCB1/P-glycoprotein. Previously, through the use of an extensive screening process, we found that vardenafil, a phosphodiesterase 5 (PDE-5) inhibitor significantly reverses MDR in ABCB1 overexpressing cancer cells, and its efficacy was greater than that of tadalafil, another PDE-5 inhibitor. The present study was designed to determine the reversal mechanisms of vardenafil and tadalafil on ABC transporters-mediated MDR. Vardenafil or tadalafil alone, at concentrations up to 20 μM, had no significant toxic effects on any of the cell lines used in this study, regardless of their membrane transporter status. However, vardenafil when used in combination with anticancer substrates of ABCB1, significantly potentiated their cytotoxicity in ABCB1 overexpressing cells in a concentration-dependent manner, and this effect was greater than that of tadalafil. The sensitivity of the parenteral cell lines to cytotoxic anticancer drugs was not significantly altered by vardenafil. The differential effects of vardenafil and tadalafil appear to be specific for the ABCB1 transporter as both vardenafil and tadalafil had no significant effect on the reversal of drug resistance conferred by ABCC1 (MRP1) and ABCG2 (BCRP) transporters. Vardenafil significantly increased the intracellular accumulation of [(3)H]-paclitaxel in the ABCB1 overexpressing KB-C2 cells. In addition, vardenafil significantly stimulated the ATPase activity of ABCB1 and inhibited the photolabeling of ABCB1 with [(125)I]-IAAP. Furthermore, Western blot analysis indicated the incubation of cells with either vardenafil or tadalafil for 72 h did not alter ABCB1 protein expression. Overall, our results suggest that vardenafil reverses ABCB1-mediated MDR by directly blocking the drug efflux function of ABCB1.     

Inhibition of multidrug resistance by adamantylgb3, a globotriaosylceramide analog

Multidrug resistance (MDR) via the ABC drug transporter (ABCB1), P-glycoprotein (P-gp/MDR1) overexpression, is a major obstacle in cancer chemotherapy. Many inhibitors reverse MDR but, like cyclosporin A (CsA), have significant toxicities. MDR1 is also a translocase that flips glucosylceramide inside the Golgi to enhance neutral glycosphingolipid (GSL) synthesis. We observed partial MDR1/globotriaosylceramide (Gb3) cell surface co-localization, and GSL removal depleted cell surface MDR1. MDR1 may therefore interact with GSLs. AdamantylGb3, a water-soluble Gb3 mimic, but not other GSL analogs, reversed MDR1-MDCK cell drug resistance. Cell surface MDR1 was up-regulated 1 h after treatment with CsA or adaGb3, but at 72 h, cell surface expression was lost. Intracellular MDR1 accumulated throughout, suggesting long term defects in plasma membrane MDR1 trafficking. AdaGb3 or CsA rapidly reduced rhodamine 123 cellular efflux. MDR1 also mediates gastrointestinal epithelial drug efflux, restricting oral bioavailability. Vinblastine apical-to-basal transport in polarized human intestinal C2BBe1 cells was significantly increased when adaGb3 was added to both sides, or to the apical side only, comparable with verapamil, a standard MDR1 inhibitor. Disulfide cross-linking of mutant MDR1s showed no binding of adaGb3 to the MDR1 verapamil/cyclosporin-binding site between surface proximal helices of transmembrane segments (TM) 6 and TM7, but rather to an adjacent site nearer the center of TM6 and the TM7 extracellular face, i.e. close to the bilayer leaflet interface. Verotoxin-mediated Gb3 endocytosis also up-regulated total MDR1 and inhibited drug efflux. Thus, a functional interplay between membrane Gb3 and MDR1 provides a more physiologically based approach to MDR1 regulation to increase the bioavailability of chemotherapeutic drugs.     

MDR1 P-glycoprotein reduces influx of substrates without affecting membrane potential

MDR1 (multidrug resistance) P-glycoprotein (Pgp; ABCB1) decreases intracellular concentrations of structurally diverse drugs. Although Pgp is generally thought to be an efflux transporter, the mechanism of action remains elusive. To determine whether Pgp confers drug resistance through changes in transmembrane potential (E(m)) or ion conductance, we studied electrical currents and drug transport in Pgp-negative MCF-7 cells and MCF-7/MDR1 stable transfectants that were established and maintained without chemotherapeutic drugs. Although E(m) and total membrane conductance did not differ between MCF-7 and MCF-7/MDR1 cells, Pgp reduced unidirectional influx and steady-state cellular content of Tc-Sestamibi, a substrate for MDR1 Pgp, without affecting unidirectional efflux of substrate from cells. Depolarization of membrane potentials with various concentrations of extracellular K(+) in the presence of valinomycin did not inhibit the ability of Pgp to reduce intracellular concentration of Tc-Sestamibi, strongly suggesting that the drug transport activity of MDR1 Pgp is independent of changes in E(m) or total ion conductance. Tetraphenyl borate, a lipophilic anion, enhanced unidirectional influx of Tc-Sestamibi to a greater extent in MCF-7/MDR1 cells than in control cells, suggesting that Pgp may, directly or indirectly, increase the positive dipole potential within the plasma membrane bilayer. Overall, these data demonstrate that changes in E(m) or macroscopic conductance are not coupled with function of Pgp in multidrug resistance. The dominant effect of MDR1 Pgp in this system is reduction of drug influx, possibly through an increase in intramembranous dipole potential.     

Psoralen reverses docetaxel-induced multidrug resistance in A549/D16 human lung cancer cells lines

Chemotherapy is the recommended treatment for advanced-stage cancers. However, the emergence of multidrug resistance (MDR), the ability of cancer cells to become simultaneously resistant to different drugs, limits the efficacy of chemotherapy. Previous studies have shown that herbal medicine or natural food may be feasible for various cancers as potent chemopreventive drug. This study aims to explore the capablility of reversing the multidrug resistance of docetaxel (DOC)-resistant A549 cells (A549/D16) of psoralen and the underlying mechanisms. In this study, results showed that the cell viability of A549/D16 subline is decreased when treated with psoralen plus DOC, while psoralen has no effect on the cell proliferation on A549 and A549/D16 cells. Furthermore, mRNA and proteins levels of ABCB1 were decreased in the presence of psoralen, while decreased ABCB1 activity was also revealed by flow cytometry. Based on these results, we believe that psoralen may be feasible for reversing the multidrug resistance by inhibiting ABCB1 gene and protein expression. Such inhibition will lead to a decrease in ABCB1 activity and anti-cancer drug efflux, which eventually result in drug resistance reversal and therefore, sensitizing drug-resistant cells to death in combination with chemotherapeutic drugs.     

Screening Compounds with a Novel High-Throughput ABCB1-Mediated Efflux Assay Identifies Drugs with Known Therapeutic Targets at Risk for Multidrug Resistance Interference

ABCB1, also known as P-glycoprotein (P-gp) or multidrug resistance protein 1 (MDR1), is a membrane-associated multidrug transporter of the ATP-binding cassette (ABC) transporter family. It is one of the most widely studied transporters that enable cancer cells to develop drug resistance. Reliable high-throughput assays that can identify compounds that interact with ABCB1 are crucial for developing new therapeutic drugs. A high-throughput assay for measuring ABCB1-mediated calcein AM efflux was developed using a fluorescent and phase-contrast live cell imaging system. This assay demonstrated the time- and dose-dependent accumulation of fluorescent calcein in ABCB1-overexpressing KB-V1 cells. Validation of the assay was performed with known ABCB1 inhibitors, XR9576, verapamil, and cyclosporin A, all of which displayed dose-dependent inhibition of ABCB1-mediated calcein AM efflux in this assay. Phase-contrast and fluorescent images taken by the imaging system provided additional opportunities for evaluating compounds that are cytotoxic or produce false positive signals. Compounds with known therapeutic targets and a kinase inhibitor library were screened. The assay identified multiple agents as inhibitors of ABCB1-mediated efflux and is highly reproducible. Among compounds identified as ABCB1 inhibitors, BEZ235, BI 2536, IKK 16, and ispinesib were further evaluated. The four compounds inhibited calcein AM efflux in a dose-dependent manner and were also active in the flow cytometry-based calcein AM efflux assay. BEZ235, BI 2536, and IKK 16 also successfully inhibited the labeling of ABCB1 with radiolabeled photoaffinity substrate [¹²⁵I]iodoarylazidoprazosin. Inhibition of ABCB1 with XR9576 and cyclosporin A enhanced the cytotoxicity of BI 2536 to ABCB1-overexpressing cancer cells, HCT-15-Pgp, and decreased the IC₅₀ value of BI 2536 by several orders of magnitude. This efficient, reliable, and simple high-throughput assay has identified ABCB1 substrates/inhibitors that may influence drug potency or drug-drug interactions and predict multidrug resistance in clinical treatment.     

Novel approaches to reversing anti-cancer drug resistance using gene-specific therapeutics.

One of the underlying mechanisms of multidrug resistance (MDR) is cellular overproduction of P-glycoprotein (P-gp), which acts as an efflux pump for various anti-cancer drugs. P-gp is encoded by a group of related genes termed MDR; only MDR1 is known to confer the drug resistance, and its overexpression in cancer cells has been a therapeutic target to circumvent the resistance. To overcome P-gp-mediated drug resistance, we have developed six anti-MDR1 hammerhead ribozymes and delivered them to P-gp-overproducing human leukemia cell line by a retroviral vector containing RNA polymerase III promoter. These ribozyme-transduced cells became vincristine-sensitive, concomitant with the decreases in MDR1 expression, P-gp amount and efflux pump function. Among the ribozymes tested, the anti-MDR1 ribozyme against the translation-initiation site exhibited the highest efficacy. The retrovirus-mediated transfer of this most potent anti-MDR1 ribozyme into a human lymphoma cell line, which was made resistant by infection of pHaMDR1/A retroviral vector and thus possessed a low degree of MDR due to P-gp expression relevant to clinical MDR, resulted in a complete reversal of MDR phenotype. In addition to retrovirus-mediated transfer of ribozymes, we evaluated the efficacy of cationic liposome-mediated transfer of ribozyme. Treatment of a P-gp-producing human breast cancer cell line with the liposome-ribozyme complex resulted in reversal of resistance, concomitant with the decreases in both MDR1 expression and P-gp amount. Confocal microscopic imaging of the cells after treatment with liposome/FITC-dextran showed cytoplasmic fluorescence that was abolished by cytochalasin B, indicating a high endocytotic activity in these cells. The endocytotic activity was well correlated with the success of cationic liposome-mediated transfer of MDR1 ribozyme. These distinct approaches using either retrovirus- or liposome-mediated transfer of anti-MDR1 ribozyme may be selectively applicable to the treatment of MDR cells with different properties such as endocytotic activity as a specific means to reverse resistance.     

Localization and Substrate Selectivity of Sea Urchin Multidrug (MDR) Efflux Transporters

In this study, we cloned, expressed and functionally characterized Stronglycentrotus purpuratus (Sp) ATP-binding cassette (ABC) transporters. This screen identified three multidrug resistance (MDR) transporters with functional homology to the major types of MDR transporters found in humans. When overexpressed in embryos, the apical transporters Sp-ABCB1a, ABCB4a, and ABCG2a can account for as much as 87% of the observed efflux activity, providing a robust assay for their substrate selectivity. Using this assay, we found that sea urchin MDR transporters export canonical MDR susbtrates such as calcein-AM, bodipy-verapamil, bodipy-vinblastine, and mitoxantrone. In addition, we characterized the impact of nonconservative substitutions in the primary sequences of drug binding domains of sea urchin versus murine ABCB1 by mutation of Sp-ABCB1a and treatment of embryos with stereoisomeric cyclic peptide inhibitors (QZ59 compounds). The results indicated that two substitutions in transmembrane helix 6 reverse stereoselectivity of Sp-ABCB1a for QZ59 enantiomers compared with mouse ABCB1a. This suggests that subtle changes in the primary sequence of transporter drug binding domains could fine-tune substrate specificity through evolution.     

Structure and function of ABCG2-rich extracellular vesicles mediating multidrug resistance

Multidrug resistance (MDR) is a major impediment to curative cancer chemotherapy. The ATP-Binding Cassette transporters ABCG2, ABCB1 and ABCC2 form a unique defense network against multiple structurally and functionally distinct chemotherapeutics, thereby resulting in MDR. Thus, deciphering novel mechanisms of MDR and their overcoming is a major goal of cancer research. Recently we have shown that overexpression of ABCG2 in the membrane of novel extracellular vesicles (EVs) in breast cancer cells results in mitoxantrone resistance due to its dramatic sequestration in EVs. However, nothing is known about EVs structure, biogenesis and their ability to concentrate multiple antitumor agents. To this end, we here found that EVs are structural and functional homologues of bile canaliculi, are apically localized, sealed structures reinforced by an actin-based cytoskeleton and secluded from the extracellular milieu by the tight junction proteins occludin and ZO-1. Apart from ABCG2, ABCB1 and ABCC2 were also selectively targeted to the membrane of EVs. Moreover, Ezrin-Radixin-Moesin protein complex selectively localized to the border of the EVs membrane, suggesting a key role for the tethering of MDR pumps to the actin cytoskeleton. The ability of EVs to concentrate and sequester different antitumor drugs was also explored. Taking advantage of the endogenous fluorescence of anticancer drugs, we found that EVs-forming breast cancer cells display high level resistance to topotecan, imidazoacridinones and methotrexate via efficient intravesicular drug concentration hence sequestering them away from their cellular targets. Thus, we identified a new modality of anticancer drug compartmentalization and resistance in which multiple chemotherapeutics are actively pumped from the cytoplasm and highly concentrated within the lumen of EVs via a network of MDR transporters differentially targeted to the EVs membrane. We propose a composite model for the structure and function of MDR pump-rich EVs in cancer cells and their ability to confer multiple anticancer drug resistance.     

N-Substituted piperazine derivatives as potential multitarget agents acting on histamine H3 receptor and cancer resistance proteins

Taking into account that multidrug resistance (MDR) is the main cause for chemotherapeutic failure in cancer treatment, the ability of novel histamine H₃ receptor ligands to reverse the cancer MDR was evaluated, using the ABCB1 efflux pump inhibition assay in mouse MDR T-lymphoma cells. The most active compounds displayed significant cytotoxic and antiproliferative effects as well as a very potent MDR efflux pump inhibitory action, 3–5-fold stronger than that of reference inhibitor verapamil. Although these compounds possess weak antagonistic properties against histamine H₃ receptors, they are valuable pharmacological tools in the search for novel anticancer molecules. Furthermore, for the most active compounds, an insight into mechanisms of action using either, the luminescent Pgp-Glo™ Assay in vitro or docking studies to human Pgp, was performed.     

Acridones circumvent P-glycoprotein-associated multidrug resistance (MDR) in cancer cells

Multidrug resistance (MDR) mediated by overexpression of MDR1 P-glycoprotein (P-gp) is one of the best characterized transporter-mediated barriers to successful chemotherapy in cancer patients. Chemosensitizers are the agents that increase the sensitivity of multidrug-resistant cells to the toxic influence of previously less effective drugs. In an attempt to find such vital chemosensitizers, a series of N(10)-substituted-2-chloroacridone analogous (1-17) have been synthesized. Compound 1 was prepared by the Ullmann condensation of o-chlorobenzoic acid and p-chloroaniline followed by cyclization. The N-(omega-chloroalkyl) analogues were found to undergo iodide catalyzed nucleophilic substitution reaction with secondary amines and the resultant products were characterized by spectral methods. The lipophilicity expressed in log(10)P and pK(a) of compounds has been determined. All compounds were examined for their ability to increase the uptake of vinblastine (VLB) in MDR KBCh(R)-8-5 cells and the results showed that the compounds 6, 8, 11-14, 16, and 17 at their respective IC(50) concentrations caused a 1.0- to 1.7-fold greater accumulation of VLB than did a similar concentration of the standard modulator, verapamil (VRP). Results of the efflux experiment showed that VRP and each of the modulators significantly inhibited the efflux of VLB, suggesting that they may be competitors for P-gp. All modulators effectively competing with [(3)H]azidopine for binding to P-gp pointed out this transport membrane protein as their likely site of action. Compounds at IC(10) were evaluated for their efficacy to modulate the cytotoxicity of VLB and the results showed that modulators 11, 13, 14, 16, and 17 were able to completely reverse the 25-fold resistance of KBCh(R)-8-5 cells to VLB. Examination of the relationship between lipophilicity and antagonism of MDR showed a reasonable correlation suggesting that hydrophobicity is one of the determinants of potency for anti-MDR activity of 2-chloroacridones. The results allowed us to draw preliminary conclusions about structural features of 2-chloroacridones important for MDR modulation.     

miR‐495 sensitizes MDR cancer cells to the combination of doxorubicin and taxol by inhibiting MDR1 expression

MDR1 is highly expressed in MDR A2780DX5 ovarian cancer cells, MDR SGC7901R gastric cancer cells and recurrent tumours. It pumps cytoplasmic agents out of cells, leading to decreased drug accumulation in cells and making cancer cells susceptible to multidrug resistance. Here, we identified that miR‐495 was predicted to target ABCB1, which encodes protein MDR1. To reduce the drug efflux and reverse MDR in cancer cells, we overexpressed a miR‐495 mimic in SGC7901R and A2780DX cells and in transplanted MDR ovarian tumours in vivo. The results indicated that the expression of MDR1 in the above cells or tumours was suppressed and that subsequently the drug accumulation in the MDR cells was decreased, cell death was increased, and tumour growth was inhibited after treatment with taxol‐doxorubicin, demonstrating increased drug sensitivity. This study suggests that pre‐treatment with miR‐495 before chemotherapy could improve the curative effect on MDR1‐based MDR cancer.     

Anti-AIDS agents 89. Identification of DCX derivatives as anti-HIV and chemosensitizing dual function agents to overcome P-gp-mediated drug resistance for AIDS therapy

In this study, 19 dicamphanoyl-dihydropyranochromone (DCP) and dicamphanoyl-dihydropyranoxanthone (DCX) derivatives, previously discovered as novel anti-HIV agents, were evaluated for their potential to reverse multi-drug resistance (MDR) in a cancer cell line over-expressing P-glycoprotein (P-gp). Seven compounds fully reversed resistance to vincristine (VCR) at 4 μM, a 20-fold enhancement compared to the first generation chemosensitizer, verapamil (4 μM). The mechanism of action of DCPs and DCXs was also resolved, since the most active compounds (3, 4, and 7) significantly increased intracellular drug accumulation due, in part, to inhibiting the P-gp mediated drug efflux from cells. We conclude that DCPs (3 and 4) and DCXs (7, 11, and 17) can exhibit polypharmacologic behavior by acting as dual inhibitors of HIV replication and chemoresistance mediated by P-gp. As such, they may be useful in combination therapy to overcome P-gp-associated drug resistance for AIDS treatment.     

A dual PI3 kinase/mTOR inhibitor BEZ235 reverses doxorubicin resistance in ABCB1 overexpressing ovarian and pancreatic cancer cell lines

BackgroundMulti-drug resistance (MDR) develops because cancer cells evade toxicity of several structurally unrelated drugs. Besides other mechanisms, MDR is linked to the overexpression of ATP Binding Cassette (ABC), transporters, among which ABCB1 is the best characterized one. Since overactivation of PI3K/Akt/mTOR plays a pivotal role in the growth of human cancers, we hypothesized whether dual PI3K and mTOR inhibitor, BEZ235 (BEZ, dactolisib) reverses resistance to doxorubicin (DOX).MethodsOvarian (A2780) and pancreatic (MiaPaca2) cancer cells were used to generate DOX-resistant clones by overexpressing ABCB1 or stepwise treatment of DOX. Intracellular accumulation of DOX was measured by flow cytometry after treatment with BEZ.ResultsBEZ treatment caused an increase in intracellular levels of DOX which was almost identical to the naïve parental cell lines. BEZ was found to be a weak substrate for ABCB1 as demonstrated by minimal increase in ATPase activity. BEZ treatment caused a dose-dependent decrease in cell viability in combination with DOX, which was associated with an increase in cleaved PARP expression in the drug resistant clones.ConclusionsThese results suggest that BEZ is a non-substrate inhibitor of ABCB1 and is able to effectively re-sensitize cells overexpressing ABCB1 to the effects of DOX.General significanceDual PI3 Kinase/mTOR inhibitor, BEZ, has the potential to reverse MDR in cancer patients.     

Induction of the multixenobiotic/multidrug resistance system in HeLa cells in response to imidazolium ionic liquids

The multixenobiotic/multidrug resistance (MXR/MDR) system controls transport of foreign molecules across the plasma membrane as a preventive measure before toxicity becomes apparent. The system consists of an efflux pump, ABCB1, and/or a member of the ABCC family. Ionic liquids are broadly used solvents with several unique properties such as wide liquid range, negligible vapor pressure, good thermal and chemical stability and extraordinary dissolution properties for organic and inorganic compounds. Ionic liquids containing imidazolium ring are frequently used as solvents in drug synthesis. Constitutive and induced amounts of ABCB1 and ABCC1 proteins were estimated here by Western blotting and quantified by flow cytometry in HeLa cells exposed to three homologous 1-alkyl-3-methylimidazolium and one benzyl ring substituted salts. Aliphatic substituents in position 1 of the salts caused a weak toxicity but 1-benzyl ring was strongly toxic. An 8-day long treatment with 10(-4) M 1-hexyl-3-methylimidazolium chloride resulted in an about 1.5-fold increase of ABCB1 level and over 2-fold increase of ABCC1 level. The amounts of both investigated ABC-proteins were linearly dependent on the length of the imidazolium ring side chain. Such distinctive changes of the amount of MXR/MDR proteins measured in cultured cells may be a useful marker when screening for potential toxicity of various chemicals.     

Mechanisms and strategies to overcome multiple drug resistance in cancer

One of the major problems in chemotherapy is multidrug resistance (MDR) against anticancer drugs. ATP-binding cassette (ABC) transporters are a family of proteins that mediate MDR via ATP-dependent drug efflux pumps. Many MDR inhibitors have been identified, but none of them have been proven clinically useful without side effects. Efforts continue to discover not toxic MDR inhibitors which lack pharmacokinetic interactions with anticancer drugs. Novel approaches have also been designed to inhibit or circumvent MDR. In this review, the structure and function of ABC transporters and development of MDR inhibitors are described briefly including various approaches to suppress MDR mechanisms.     

Role of the highly structured 5'-end region of MDR1 mRNA in P-glycoprotein expression

Overexpression of P-glycoprotein, encoded by the MDR1 (multidrug resistance 1) gene, is often responsible for multidrug resistance in acute myeloid leukaemia. We have shown previously that MDR1 (P-glycoprotein) mRNA levels in K562 leukaemic cells exposed to cytotoxic drugs are up-regulated but P-glycoprotein expression is translationally blocked. In the present study we show that cytotoxic drugs down-regulate the Akt signalling pathway, leading to hypophosphorylation of the translational repressor 4E-BP [eIF (eukaryotic initiation factor) 4E-binding protein] and decreased eIF4E availability. The 5'-end of MDR1 mRNA adopts a highly-structured fold. Fusion of this structured 5'-region upstream of a reporter gene impeded its efficient translation, specifically under cytotoxic stress, by reducing its competitive ability for the translational machinery. The effect of cytotoxic stress could be mimicked in vivo by blocking the phosphorylation of 4E-BP by mTOR (mammalian target of rapamycin) using rapamycin or eIF4E siRNA (small interfering RNA), and relieved by overexpression of either eIF4E or constitutively-active Akt. Upon drug exposure MDR1 mRNA was up-regulated, apparently stochastically, in a small proportion of cells. Only in these cells could MDR1 mRNA compete successfully for the reduced amounts of eIF4E and translate P-glycoprotein. Consequent drug efflux and restoration of eIF4E availability results in a feed-forward relief from stress-induced translational repression and to the acquisition of drug resistance.     

Axitinib targeted cancer stemlike cells to enhance efficacy of chemotherapeutic drugs via inhibiting the drug transport function of ABCG2

Stemlike cells have been isolated by their ability to efflux Hoechst 33342 dye and are called the side population (SP). We evaluated the effect of axitinib on targeting cancer stemlike cells and enhancing the efficacy of chemotherapeutical agents. We found that axitinib enhanced the cytotoxicity of topotecan and mitoxantrone in SP cells sorted from human lung cancer A549 cells and increased cell apoptosis induced by chemotherapeutical agents. Moreover, axitinib particularly inhibited the function of adenosine triphosphate (ATP)-binding cassette subfamily G member 2 (ABCG2) and reversed ABCG2-mediated multidrug resistance (MDR) in vitro. However, no significant reversal effect was observed in ABCB1-, ABCC1- or lung resistance-related protein (LRP)-mediated MDR. Furthermore, in both sensitive and MDR cancer cells axitinib neither altered the expression of ABCG2 at the mRNA or protein levels nor blocked the phosphorylation of AKT and extracellular signal-regulated kinase (ERK)1/2. In nude mice bearing ABCG2-overexpressing S1-M1-80 xenografts, axitinib significantly enhanced the antitumor activity of topotecan without causing additional toxicity. Taken together, these data suggest that axitinib particularly targets cancer stemlike cells and reverses ABCG2-mediated drug resistance by inhibiting the transporter activity of ABCG2.     

Genes of multidrug resistance in haematological malignancies

Since the early 1970s, multiple drug resistance has been known to exist in cancer cells and is thought to be attributable to a membrane-bound, energy-dependent pump protein (P-glycoprotein [P-gp]) capable of extruding various related and unrelated chemotherapeutic drugs. The development of refractory disease in haematological malignancies is frequently associated with the expression of one or several multidrug resistance (MDR) genes. MDR1, multidrug resistance-associated protein (MRP) and lung-resistance protein (LRP) have been identified as important adverse prognostic factors. Recently it has become possible to reverse clinical MDR by blocking P-gp-mediated drug efflux. The potential relevance of these reversal agents of MDR as well as the potential new approaches to treat the refractory disease are discussed in this article. In addition, an array of different molecules and mechanisms by which resistant cells can escape the cytotoxic effect of anticancer drugs has now been identified. These molecules and mechanisms include apoptosis-related proteins and drug inactivation enzymes. Resistance to chemotherapy is believed to cause treatment failure in more than 50% patients. Clearly, if drug resistance could be overcome, the impact on survival would be highly significant. This review focuses on molecular mechanism of drug resistance in haematological malignancies with emphasis on molecules involved in MDR. In addition, it brings the survey of methods involved in determination of MDR, in particular P-gp/MDR1, MRP and LRP.